1. Field of the Invention
The present invention relates to an oven controlled crystal oscillator (OCXO), and particularly to an oven controlled crystal oscillator having stable frequency-temperature characteristics.
2. Description of the Related Art
The oven controlled crystal oscillator is a crystal oscillator having a crystal resonator mounted in a case in which temperature is kept constant. Since the oven controlled crystal oscillator is less likely to be affected by environmental temperature, it has stable frequency characteristics.
The structure of a conventional oven controlled crystal oscillator will be described with reference to FIGS. 4 and 5. FIG. 4 is a sectional view showing the structure of the conventional oven controlled crystal oscillator. FIG. 5 is an explanatory drawing showing the structure of a substrate 1 in the conventional oven controlled crystal oscillator, where (a) is a plan view, (b) is a top view, (c) is a left side view, (d) is a bottom view, and (e) is a right side view.
As shown in FIGS. 4 and 5, in the conventional oven controlled crystal oscillator (hereinafter referred to as “OCXO”), the substrate 1 with various electronic components such as a crystal resonator mounted thereon is provided in a space enclosed by a cover 7 and a base 8 in such a manner to be fixed to the base 8 with pins 9.
A surface-mounted crystal resonator 2, a temperature sensor 5, resistors 3, and a transistor 4 are provided on one side of the substrate 1 (on the side to face the base 8 in FIG. 4), and other electronic components 6 such as an oscillation circuit and a temperature control circuit are provided on the other side of the substrate 1. Note that some of the electronic components 6 are also provided on the same side as the crystal resonator 2. The resistors 3 and the transistor 4 form a heat generator for generating heat.
Then, as shown in FIG. 5(a), the temperature sensor 5, the resistors 3, and the transistor 4 are arranged around the crystal resonator 2, so that heat generated by the resistors 3 and the transistor 4 is transmitted to the crystal resonator 2 and the temperature sensor 5 through the substrate 1 and substrate wiring.
A conventional technique for implementing the oven controlled crystal oscillator is disclosed in Japanese Patent Application Laid-Open No. 11-214929 (Applicant: TOYO Communication Equipment Co., Ltd.; Inventor: Tomio Sato) laid open on Aug. 6, 1999 and entitled “Piezoelectric Oscillator.”
In this conventional technique, an oscillation circuit, a plurality of surface-mounted heaters, and a temperature control circuit are arranged on a substrate to heat the case of a piezoelectric resonator and a lead terminal portion of the piezoelectric resonator simultaneously by different heaters in order to heat the piezoelectric resonator, the oscillation circuit, and the temperature control circuit simultaneously, enabling to provide a crystal oscillator with excellent starting characteristics and warm-up time characteristics.
As other conventional techniques for implementing the oven controlled crystal oscillator, there are disclosed in Japanese Patent Application Laid-Open No. 2001-127579, No. 2003-224425, and No. 2005-117189.
However, since the structures of the conventional oven controlled crystal oscillators have a problem that heat generated by the heat generator consisting such as of the resistors 3 and the transistor 4 is hard to be transmitted uniformly over the substrate, a temperature difference can be produced between the temperature sensor 5 and the crystal resonator 2 or any of the other electronic components. In such a case, reliable temperature control cannot be performed, resulting in degradation of the frequency-temperature characteristics of the output frequency.